The group feature allows you to select files and folders you'd like to burn together on 1 CD/DVD/other medium. BTTB can pre-group items based on filename matching and MP3 tag genres. You can change these rules with Define group rules, which will show the Group settings dialog.

On the left you can view and alter the groups BTTB has found based on the group rules you have defined and/or MP3 genre tags.

Use Add to create an empty new group you can put items in, and Disband group to remove it and place all files in the right (individual files/folders) pane.

You can drag and drop items between the windows to move them in or out of groups. Besides dragging you can also move the currently selected items out of groups by pressing Move out of group. Move into Current or New group do the opposite.

Groups which fit on the medium you requested have a Green CD icon. Groups too large to fit will have a Red exploding CD icon

You can checkmark items you want to burn immediately, on the first CD. BTTB will search through the other (unchecked) items to fill the remainder of the CD. Keep an eye on the Selected for direct burning indicator to see whether everything still fits on the CD size you selected- of course, the more space you leave, the easier BTTB can fill the CD to the maximum.